1. Field of the Invention
The present invention relates to a seat belt device, and more particularly, to a seat belt device provided with an energy absorption mechanism that eases the load that an occupant receives from a webbing during a vehicle collision.
2. Related Technology
Seat belt devices for holding the occupant of a vehicle in his/her seat are provided with a emergency lock mechanism that is driven by a sensor that reacts to sudden acceleration, shocks or deceleration, so that, as a result, the emergency lock mechanism physically locks the drawing out of the webbing. The occupant can be thus effectively and safely restrained.
In order to enhance occupant protection, the latest seat belt devices are provided with a pretensioner for removing webbing slack during an emergency, such as a vehicle collision. As a result, the occupant is reliably held fast before moving frontward. An explosive is ordinarily used for actuating the pretensioner. The gas released upon blowup of the explosive causes the webbing to be quickly drawn in. A energy absorption mechanism is provided that absorbs energy and reduces the load on the chest of the occupant, while allowing for a predetermined extent of webbing draw-out, when the collision force in a vehicle collision is substantial and the load acting on the webbing on account of the inertial movement of the occupant becomes equal to or higher than a predetermined value established beforehand.
Ordinarily, torsion bars are widely employed as an energy absorption mechanism in such seat belt devices. A torsion bar is disposed coaxially with a spindle that takes the webbing up, such that one end of the torsion bar is fixed to the spindle, and the other end is fixed to a lock member (a component also referred to as tread head) of the emergency lock mechanism. When the load acting on the spindle in a webbing draw-out direction becomes equal to or higher than predetermined value during operation of the emergency lock mechanism, the torsion bar undergoes torsional deformation, and absorbs thereby collision energy (see, for instance, Japanese Patent Application Laid-open No. 2007-69686).
The spindle and the lock member of the emergency lock mechanism must be connected so as to rotate integrally with each other at an initial state. That is because if the above two members are not connected so as to rotate integrally, usability becomes impaired upon ordinary take-up or draw-out of the webbing, and noise and backlash occur as a result. For instance, pins are ordinarily used as a connecting means in such cases.
Coupling between the spindle and the lock member by way of pins is advantageous in that even in a strong connection, once the pins are sheared, the pin-related load is not compounded with the energy absorption load of the torsion bar (hereafter, EA load). However, the load may rise instantaneously during shearing, and the breaking load may become compounded with the EA load at that time.
Conventionally, the breaking load of the pins has been smaller than the EA load, even if the breaking load of the pin is compounded with the EA load, and hence no particular problems occurred. In recent years, however, the EA load has decreased in order to improve the collision absorption performance of vehicle bodies, and to improve the performance of occupant restraining techniques. Accordingly, the breaking load of the pins may now be greater than the EA load in some cases.